Tauopathies: Mechanisms and Therapeutic Strategies

Uncovering Cell Cycle Dysregulations and Associated Mechanisms in Cancer and Neurodegenerative Disorders: A Glimpse of Hope for Repurposed Drugs

The cell cycle is the sequence of events orchestrated by a complex network of cell cycle proteins. Unlike normal cells, mature neurons subsist in a quiescent state of the cell cycle, and aberrant cell cycle activation triggers neuronal death accompanied by neurodegeneration. The periodicity of cell cycle events is choreographed by various mechanisms, including DNA damage repair, oxidative stress, neurotrophin activity, and ubiquitin-mediated degradation. Given the relevance of cell cycle processes in cancer and neurodegeneration, this review delineates the overlapping cell cycle events, signaling pathways, and mechanisms associated with cell cycle aberrations in cancer and the major neurodegenerative disorders. We suggest that dysregulation of some common fundamental signaling processes triggers anomalous cell cycle activation in cancer cells and neurons. We discussed the possible use of cell cycle inhibitors for neurodegenerative disorders and described the associated challenges. We propose that a greater understanding of the common mechanisms driving cell cycle aberrations in cancer and neurodegenerative disorders will open a new avenue for the development of repurposed drugs.

Evaluation of Cell-Specific Alterations in Alzheimer's Disease and Relevance of In Vitro Models

Alzheimer's disease (AD) is a neurodegenerative disorder classically characterized by two neuropathological hallmarks: β-amyloid plaques and tau tangles in the brain. However, the cellular and molecular mechanisms involved in AD are still elusive, which dampens the possibility of finding new and more effective therapeutic interventions. Current in vitro models are limited in modelling the complexity of AD pathogenesis. In this study, we aimed to characterize the AD expression signature upon a meta-analysis of multiple human datasets, including different cell populations from various brain regions, and compare cell-specific alterations in AD patients and in vitro models to highlight the appropriateness and the limitations of the currently available models in recapitulating AD pathology. The meta-analysis showed consistent enrichment of the Rho GTPases signaling pathway among different cell populations and in the models. The accuracy of in vitro models was higher for neurons and lowest for astrocytes. Our study underscores the particularly low fidelity in modelling down-regulated genes across all cell populations. The top enriched pathways arising from meta-analysis of human data differ from the enriched pathways arising from the overlap. We hope that our data will prove useful in indicating a starting point in the development of future, more complex, 3D in vitro models.

Neuronal accumulation of hyperphosphorylated tau protein predicts stable memory impairment in people living with HIV

OBJECTIVES

As lifespans increase in people with HIV (PWH), there is concern that age-related neurodegenerative disorders may contribute to cognitive decline. We asked whether brain accumulation of Alzheimer's disease (AD)-associated proteins amyloid-beta (Aβ) and hyperphosphorylated tau (p-tau) predicted cognitive performance in middle-aged PWH.

METHODS

In a prospectively followed, cognitively-characterized autopsy sample of 135 PWH, we used immunohistochemistry to assess Aβ plaques and neuronal p-tau in medial temporal and lateral frontal lobes. These pathologies were tested for associations with cognitive performance in seven domains: motor, speed of information processing, working memory, memory encoding, memory retrieval, verbal fluency, and abstraction/executive function. Univariate and multivariate analyses accounting for HIV-associated variables, reading level, and comorbidities were conducted. Longitudinal trajectories of memory functions were evaluated in 60 individuals with a median follow-up of 6.0 years.

RESULTS

In this population with mean age 51.4 ± 0.9 years, 58% displayed neuronal p-tau and 29% Aβ plaques. Neuronal p-tau, but not Aβ, predicted worse memory encoding and retrieval, but not other cognitive functions. With an ordinal hierarchy of neuronal p-tau locations (entorhinal, hippocampal, neocortical), decreased memory performance correlated with neocortical distribution. Memory function trajectories could not be distinguished between individuals with and without neuronal p-tau, and over 80% of the sample showed no change over time.

CONCLUSION

In this middle-aged sample, neuronal p-tau accumulation contributes to memory deficits, but is not associated with accelerated decline in function over time. In the absence of AD-like deterioration, other etiologies for neuronal p-tau in cognitively impaired PWH must be considered.

Alzheimer's Disease Pathology in Middle Aged and Older People with HIV: Comparisons with Non-HIV Controls on a Healthy Aging and Alzheimer's Disease Trajectory and Relationships with Cognitive Function

We determined the prevalence of Alzheimer's disease (AD) pathological hallmarks, amyloid-β and phosphorylated-Tau, in autopsied brains of 49 people with HIV (PWH) (ages: 50-68; mean age = 57.0) from the National NeuroAIDS Tissue Consortium and in a comparative cohort of 55 people without HIV (PWoH) from the UC San Diego Alzheimer's Disease Research Center (17 controls, 14 mild cognitive impairment, 24 AD; ages: 70-102, mean age = 88.7). We examined how AD pathology relates to domain-specific cognitive functions in PWH overall and in sex-stratified samples. Amyloid-β and phosphorylated-Tau positivity (presence of pathology of any type/density) was determined via immunohistochemistry in AD-sensitive brain regions. Among PWH, amyloid-β positivity ranged from 19% (hippocampus) to 41% (frontal neocortex), and phosphorylated-Tau positivity ranged from 47% (entorhinal cortex) to 73% (transentorhinal cortex). Generally, AD pathology was significantly less prevalent, and less severe when present, in PWH versus PWoH regardless of cognitive status. Among PWH, positivity for AD pathology related most consistently to memory-related domains. Positivity for p-Tau pathology related to memory-related domains in women with HIV only, although the sample size of women with HIV was small (n = 10). Results indicate that AD pathology is present in a sizable portion of middle aged and older PWH, although not to the extent in older PWoH. Studies with better age-matched PWoH are needed to examine the effect of HIV status on AD pathology.

Neurobiochemical, Peptidomic, and Bioinformatic Approaches to Characterize Tauopathy Peptidome Biomarker Candidates in Experimental Mouse Model of Traumatic Brain Injury

Traumatic brain injury (TBI) is a multidimensional damage, and currently, no FDA-approved medicine is available. Multiple pathways in the cell are triggered through a head injury (e.g., calpain and caspase activation), which truncate tau and generate variable fragment sizes (MW 400-45,000 K). In this study, we used an open-head TBI mouse model generated by controlled cortical impact (CCI) and collected ipsilateral (IC) and contralateral (CC) mice htau brain cortices at one (D1) three (D3), and seven (D7) days post-injury. We implemented immunological (antibody-based detection) and peptidomic approaches (nano-reversed-phase liquid chromatography/tandem mass spectrometry) to investigate proteolytic tau peptidome (low molecular weight (LMW) < 10 K)) and pathological phosphorylation sites (high-molecular-weight (HMW); > 10 K) derived from CCI-TBI animal models. Our immunoblotting analysis verified tau hyperphosphorylation, HMW, and HMW breakdown products (HMW-BDP) formation of tau (e.g., pSer₂₀₂, pThr₁₈₁, pThr₂₃₁, pSer₃₉₆, and pSer₄₀₄), following CCI-TBI. Peptidomic data revealed unique sequences of injury-dependent proteolytic peptides generated from human tau protein. Among the N-terminal tau peptides, EIPEGTTAEEAGIGDTPSLEDEAAGHVTQA (a.a. 96-125) and AQPHTEIPEGTTAEEAGIGDTPSLEDEAAGHVTQARM (a.a. 91-127). Examples of tau C-terminal peptides identified include NVSSTGSIDMVDSPQLATLADEVSASLAKQGL (a.a. 410-441) and QLATLADEVSASLAKQGL (a.a. 424-441). Our peptidomic bioinformatic tools showed the association of proteases, such as CAPN1, CAPN2, and CTSL; CASP1, MMP7, and MMP9; and ELANE, GZMA, and MEP1A, in CCI-TBI tau peptidome. In clinical trials for novel TBI treatments, it might be useful to monitor a subset of tau peptidome as targets for biomarker utility and use them for a "theranostic" approach.

Genome-wide association study of brain biochemical phenotypes reveals distinct genetic architecture of Alzheimer's disease related proteins

BACKGROUND

Alzheimer's disease (AD) is neuropathologically characterized by amyloid-beta (Aβ) plaques and neurofibrillary tangles. The main protein components of these hallmarks include Aβ40, Aβ42, tau, phosphor-tau, and APOE. We hypothesize that genetic variants influence the levels and solubility of these AD-related proteins in the brain; identifying these may provide key insights into disease pathogenesis.

METHODS

Genome-wide genotypes were collected from 441 AD cases, imputed to the haplotype reference consortium (HRC) panel, and filtered for quality and frequency. Temporal cortex levels of five AD-related proteins from three fractions, buffer-soluble (TBS), detergent-soluble (Triton-X = TX), and insoluble (Formic acid = FA), were available for these same individuals. Variants were tested for association with each quantitative biochemical measure using linear regression, and GSA-SNP2 was used to identify enriched Gene Ontology (GO) terms. Implicated variants and genes were further assessed for association with other relevant variables.

RESULTS

We identified genome-wide significant associations at seven novel loci and the APOE locus. Genes and variants at these loci also associate with multiple AD-related measures, regulate gene expression, have cell-type specific enrichment, and roles in brain health and other neuropsychiatric diseases. Pathway analysis identified significant enrichment of shared and distinct biological pathways.

CONCLUSIONS

Although all biochemical measures tested reflect proteins core to AD pathology, our results strongly suggest that each have unique genetic architecture and biological pathways that influence their specific biochemical states in the brain. Our novel approach of deep brain biochemical endophenotype GWAS has implications for pathophysiology of proteostasis in AD that can guide therapeutic discovery efforts focused on these proteins.

Synthesis and biological evaluation of thieno[3,2-c]pyrazol-3-amine derivatives as potent glycogen synthase kinase 3β inhibitors for Alzheimer's disease

Glycogen synthase kinase 3β (GSK-3β) catalyses the hyperphosphorylation of tau protein in the Alzheimer's disease (AD) pathology. A series of novel thieno[3,2-c]pyrazol-3-amine derivatives were designed and synthesised and evaluated as potential GSK-3β inhibitors by structure-guided drug rational design approach. The thieno[3,2-c]pyrazol-3-amine derivative 16b was identified as a potent GSK-3β inhibitor with an IC₅₀ of 3.1 nM in vitro and showed accepted kinase selectivity. In cell levels, 16b showed no toxicity on the viability of SH-SY5Y cells at the concentration up to 50 μM and targeted GSK-3β with the increased phosphorylated GSK-3β at Ser9. Western blot analysis indicated that 16b decreased the phosphorylated tau at Ser396 in a dose-dependent way. Moreover, 16b effectively increased expressions of β-catenin as well as the GAP43, N-myc, and MAP-2, and promoted the differentiated neuronal neurite outgrowth. Therefore, the thieno[3,2-c]pyrazol-3-amine derivative 16b could serve as a promising GSK-3β inhibitor for the treatment of AD.

On the Tracks of the Aggregation Mechanism of the PHF6 Peptide from Tau Protein: Molecular Dynamics, Energy, and Interaction Network Investigations

The formation of neurofibrillary tangles (NFTs), composed of tau protein aggregates, is a hallmark of some neurodegenerative diseases called tauopathies. NFTs are composed of paired helical filaments (PHFs) of tau protein with a dominant β-sheet secondary structuration. The NFT formation mechanism is not known yet. This study focuses on PHF6, a crucial hexapeptide responsible for tau aggregation. A 2 μs molecular dynamics simulation was launched to determine the keys of the PHF6 aggregation mechanism. Hydrogen bonding, van der Waals, and other non-covalent interactions as π-stacking were investigated. Parallel aggregation was slightly preferred due to its adaptability, but antiparallel aggregation remained widely present during the PHF6 aggregation. The analysis highlighted the leading role of hydrogen bonds identified at the atomic level for each aggregation process. The aggregation study emphasized the importance of Tyr310 during the β-sheets' complexation through π-stacking.

M2-Like Microglia Polarization Attenuates Neuropathic Pain Associated with Alzheimer's Disease

Many Alzheimer's disease (AD) patients suffer from persistent neuropathic pain (NP), which is mediated, at least partially, but microglia. Nevertheless, the exact underlying mechanism is unknown. Moreover, a clinically translatable approach through modulating microglia for treating AD-associated NP is not available. Here, in a doxycycline-induced mouse model (rTg4510) for AD, we showed development of NP. We found that the total number of microglia in the CA3 region was not increased, but polarized to pro-inflammatory M1-like phenotype, with concomitant increases in production and secretion of pro-inflammatory cytokines. To examine whether this microglia polarization plays an essential role in the AD-associated NP, we generated an adeno-associated virus (AAV) serotype PHP.B (capable of crossing the blood-brain barrier) carrying shRNA for DNA methyltransferase 1 (DNMT1) under a microglia-specific TMEM119 promoter (AAV-pTMEM119-shDNMT1), which specifically targeted microglia and induced a M2-like polarization in vitro and in vivo in doxycycline-treated rTg4510 mice. Intravenous infusion of AAV-pTMEM119-shDNMT1 induced M2-polarization of microglia and attenuated both AD-associated behavior impairment but also NP in the doxycycline-treated rTg4510 mice. Thus, our data suggest that AD-associated NP may be treated through M2-polarization of microglia.

The Significance of Vascular Pathogenesis in the Examination of Corticobasal Syndrome

Corticobasal syndrome (CBS) is a clinical entity, classified as an atypical Parkinsonism, characterized by both motor and higher cortical dysfunctions. The clinical manifestation of CBS is associated with several pathologies, among which corticobasal degeneration (CBD) is the most common. The aim of our study was to elaborate on the possible vascular pathogenesis of CBS and consider types of vascular lesions in these cases. Several cases of vascular CBS are described in the literature. The majority of presented patients were affected by internal carotid artery (ICA) stenosis and ischemic strokes; few cases were associated with vascular malformations or autoimmune diseases. Vascular CBS is preceded by an abrupt onset. The clinical manifestation does not significantly differ with non-vascular CBS. Patients with vascular CBS are usually elderly; often with coexistent hypertension, dyslipidemia and diabetes mellitus. Inferring from our observations, cerebral hypoperfusion can play a significant role in neuropathological changes in neurodegenerative diseases. To the best of our knowledge paper is the first comprehensive review of vascular CBS and we are positive that our observations show that further research concerning the vascular pathogenesis of tauopathy atypical Parkinsonism is required.

Screening of tau protein kinase inhibitors in a tauopathy-relevant cell-based model of tau hyperphosphorylation and oligomerization

Tauopathies are a class of neurodegenerative disorders characterized by abnormal deposition of post-translationally modified tau protein in the human brain. Tauopathies are associated with Alzheimer's disease (AD), chronic traumatic encephalopathy (CTE), and other diseases. Hyperphosphorylation increases tau tendency to aggregate and form neurofibrillary tangles (NFT), a pathological hallmark of AD. In this study, okadaic acid (OA, 100 nM), a protein phosphatase 1/2A inhibitor, was treated for 24h in mouse neuroblastoma (N2a) and differentiated rat primary neuronal cortical cell cultures (CTX) to induce tau-hyperphosphorylation and oligomerization as a cell-based tauopathy model. Following the treatments, the effectiveness of different kinase inhibitors was assessed using the tauopathy-relevant tau antibodies through tau-immunoblotting, including the sites: pSer202/pThr205 (AT8), pThr181 (AT270), pSer202 (CP13), pSer396/pSer404 (PHF-1), and pThr231 (RZ3). OA-treated samples induced tau phosphorylation and oligomerization at all tested epitopes, forming a monomeric band (46-67 kDa) and oligomeric bands (170 kDa and 240 kDa). We found that TBB (a casein kinase II inhibitor), AR and LiCl (GSK-3 inhibitors), cyclosporin A (calcineurin inhibitor), and Saracatinib (Fyn kinase inhibitor) caused robust inhibition of OA-induced monomeric and oligomeric p-tau in both N2a and CTX culture. Additionally, a cyclin-dependent kinase 5 inhibitor (Roscovitine) and a calcium chelator (EGTA) showed contrasting results between the two neuronal cultures. This study provides a comprehensive view of potential drug candidates (TBB, CsA, AR, and Saracatinib), and their efficacy against tau hyperphosphorylation and oligomerization processes. These findings warrant further experimentation, possibly including animal models of tauopathies, which may provide a putative Neurotherapy for AD, CTE, and other forms of tauopathy-induced neurodegenerative diseases.

Potassium Chloride Cotransporter 2 Inhibits Neuropathic Pain and Future Development of Neurodegeneration

Persistent neuropathic pain (NP) causes future development of neurodegenerative diseases, e.g., Alzheimer' disease, and thus needs to be optimally treated. Surgically-induced neuropathic pain (SNPP) is a persistent pain that occurs in nearly half of the individuals after common operations. Here, we showed that specific activation of 5-hydroxytryptamine (5-HT) type 2A receptors by systemic administration of TCB-2 [(4-bromo-3,6-dimethoxybenzocyclobuten-1-yl) methylamine hydrobromide] improved the function of potassium chloride cotransporter 2 (KCC2), resulting in reduction in neuropathic pain after chronic constriction injury (CCI), a rat model that mimics SNPP. Moreover, TCB-2 administration attenuated both mechanical and thermal hyperalgesia, likely through augmentation of dorsal horn KCC2 levels, since this effect was abolished by intrathecal provision of dihydroindenyl oxy alkanoic acid (DIOA), which blocked the effects of KCC2. Furthermore, TCB-2-mediated re-activation of KCC2 likely reduces future development of neurodegeneration in rats. Together, our data support further studies on the possibility of using this strategy to reduce postoperative pain and future neurodegenerative disorders in clinic.

Disease-modifying strategies in primary tauopathies

Tauopathies are neurodegenerative brain diseases that are characterized by the formation of intraneuronal inclusions containing the microtubule-associated protein tau. This major hallmark defines tau pathology which is predominant in primary tauopathies, while in secondary forms additional driving forces are involved. In the course of the disease, different brain areas degenerate and lead to severe defects of language, behavior and movement. Although neuropathologically heterogeneous, primary tauopathies share a common feature, which is the generation of abnormal tau species that aggregate and progress into filamentous deposits in neurons. Mechanisms that are involved in this disease-related process offer a broad range of targets for disease-modifying therapeutics. The present review provides an up-to-date overview of currently known targets in primary tauopathies and their possible therapeutic modulation. It is structured into four major targets, the post-translational modifications of tau and tau aggregation, protein homeostasis, disease propagation, and tau genetics. Chances, as well as obstacles in the development of effective therapies are highlighted. Some therapeutic strategies, e.g., passive or active immunization, have already reached clinical development, raising hopes for affected patients. Other concepts, e.g., distinct modulators of proteostasis, are at the ready to be developed into promising future therapies. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Novel Peptidomic Approach for Identification of Low and High Molecular Weight Tauopathy Peptides Following Calpain Digestion, and Primary Culture Neurotoxic Challenges

Tauopathy is a class of a neurodegenerative disorder linked with tau hyperphosphorylation, proteolysis, and aggregation. Tau can be subjected to proteolysis upon calpain activation in Alzheimer disease (AD), and traumatic brain injury (TBI). We and others have extensively researched calpain-mediated tau breakdown products (Tau-BDP; 45K, 35K, and 17K). Tau proteolysis might also generate low molecular weight (LMW ≤10K) proteolytic peptides after neurodegenerative damage. In this study, we have subjected purified tau protein (phospho and non-phospho) and mouse brain lysate to calpain-1 digestion to characterize the LMW generated by nano-liquid chromatography coupled to electrospray ionization to tandem mass spectrometry (nano-LC-ESI-MS/MS). We have also challenged differentiated primary cerebrocortical neuronal cultures (CTX) with neurotoxic agents (calcium ionophore calcimycin (A23187), staurosporine (STS), N-methyl-D-aspartate (NMDA), and Maitotoxin (MTX)) that mimic neurodegeneration to investigate the peptidome released into the conditioned cell media. We used a simple workflow in which we fractionate LMW calpain-mediated tau peptides by ultrafiltration (molecular weight cut-off value (MWCO) of 10K) and subject filtrate fractions to nano-LC-MS/MS analysis. The high molecular weight (HMW) peptides and intact proteins retained on the filter were analyzed separately by western blotting using total and phospho-specific tau antibodies. We have identified several novel proteolytic tau peptides (phosphorylated and non-phosphorylated) that are only present in samples treated with calpain or cell-based calpain activation model (particularly N- and C-terminal peptides). Our findings can help in developing future research strategies emphasizing on the suppression of tau proteolysis as a target.

Microglial inflammation and phagocytosis in Alzheimer's disease: Potential therapeutic targets

One of the largest unmet medical needs is a disease-modifying treatment for Alzheimer's disease (AD). Recently, the role of microglia in disease, particularly AD, has gained great interest, following the identification of several disease risk-associated genes that are highly expressed in microglia. Microglia play a critical homeostatic role in the brain, with neuroinflammatory and phagocytic mechanisms being of particular importance. Here, we review the role of NLRP3, the complement system, and the triggering receptor expressed in myeloid cells 2 (TREM2) in modulating microglial functions. We have reviewed the targets, their molecular pathways and the therapeutic interventions aimed at modulating these targets, in the hope of discovering a novel therapeutic approach for the treatment of AD. LINKED ARTICLES: This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

Neuroprotective Approach of Anti-Cancer Microtubule Stabilizers Against Tauopathy Associated Dementia: Current Status of Clinical and Preclinical Findings

Neuronal microtubule (MT) tau protein provides cytoskeleton to neuronal cells and plays a vital role including maintenance of cell shape, intracellular transport, and cell division. Tau hyperphosphorylation mediates MT destabilization resulting in axonopathy and neurotransmitter deficit, and ultimately causing Alzheimer’s disease (AD), a dementing disorder affecting vast geriatric populations worldwide, characterized by the existence of extracellular amyloid plaques and intracellular neurofibrillary tangles in a hyperphosphorylated state. Pre-clinically, streptozotocin stereotaxically mimics the behavioral and biochemical alterations similar to AD associated with tau pathology resulting in MT assembly defects, which proceed neuropathological cascades. Accessible interventions like cholinesterase inhibitors and NMDA antagonist clinically provides only symptomatic relief. Involvement of microtubule stabilizers (MTS) prevents tauopathy particularly by targeting MT oriented cytoskeleton and promotes polymerization of tubulin protein. Multiple in vitro and in vivo research studies have shown that MTS can hold substantial potential for the treatment of AD-related tauopathy dementias through restoration of tau function and axonal transport. Moreover, anti-cancer taxane derivatives and epothiolones may have potential to ameliorate MT destabilization and prevent the neuronal structural and functional alterations associated with tauopathies. Therefore, this current review strictly focuses on exploration of various clinical and pre-clinical features available for AD to understand the neuropathological mechanisms as well as introduce pharmacological interventions associated with MT stabilization. MTS from diverse natural sources continue to be of value in the treatment of cancer, suggesting that these agents have potential to be of interest in the treatment of AD-related tauopathy dementia in the future.

Four-repeat tauopathies

Tau is a microtubule-associated protein with versatile functions in the dynamic assembly of the neuronal cytoskeleton. Four-repeat (4R-) tauopathies are a group of neurodegenerative diseases defined by cytoplasmic inclusions predominantly composed of tau protein isoforms with four microtubule-binding domains. Progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease or glial globular tauopathy belong to the group of 4R-tauopathies. The present review provides an introduction in the current concept of 4R-tauopathies, including an overview of the neuropathological and clinical spectrum of these diseases. It describes the genetic and environmental etiological factors, as well as the contemporary knowledge about the pathophysiological mechanisms, including post-translational modifications, aggregation and fragmentation of tau, as well as the role of protein degradation mechanisms. Furthermore, current theories about disease propagation are discussed, involving different extracellular tau species and their cellular release and uptake mechanisms. Finally, molecular diagnostic tools for 4R-tauopathies, including tau-PET and fluid biomarkers, and investigational therapeutic strategies are presented. In summary, we report on 4R-tauopathies as overarching disease concept based on a shared pathophysiological concept, and highlight the challenges and opportunities on the way towards a causal therapy.